CN108318762B - Method for judging power frequency ferromagnetic resonance of voltage transformer - Google Patents
Method for judging power frequency ferromagnetic resonance of voltage transformer Download PDFInfo
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- G—PHYSICS
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- G—PHYSICS
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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Abstract
The method comprises the steps of firstly sampling three-phase voltage of a voltage transformer, calculating three-phase fundamental voltage and fundamental zero-sequence voltage, further judging whether the voltage transformer is in a ground fault or not when the fundamental zero-sequence voltage exceeds a set threshold, and considering that power frequency ferromagnetic resonance occurs if the voltage transformer is not in the ground fault. When the earth fault is judged, the average value of three-phase potential is calculated only by calculating the modulus of the phasor sum of each phase fundamental wave voltage and fundamental wave zero sequence voltage, when the modulus value is in a set range, the earth fault is considered to occur, and when no three phases are judged as the earth fault, the power frequency ferromagnetic resonance is considered to occur. The method is easy to realize and covers more working conditions of power frequency ferromagnetic resonance.
Description
Technical Field
The invention relates to fault monitoring of a voltage transformer of a power system, in particular to fault monitoring of ferromagnetic resonance of the voltage transformer of the power system.
Background
Electromagnetic voltage transformers (hereinafter referred to as voltage transformers) are widely used in medium-voltage and high-voltage systems of electric power systems. The voltage transformer has high excitation impedance, and after some operation processes of the power transmission line or after the ground fault disappears, a special three-phase or single-phase resonance loop is formed between the voltage transformer and the stray capacitance of the power transmission line wire to the ground or other power equipment, so that ferromagnetic resonance overvoltage which excites various harmonics (fractional harmonics, power frequency and high frequency) can occur. When resonance occurs, the voltage of one phase winding of the voltage transformer rises, or the voltages of two-phase and three-phase windings rise simultaneously, namely the voltages of all phases of the phase-to-ground change; on the other hand, however, the three-phase winding potentials of the power transformer are maintained constant. Finally, the displacement voltage appears at the neutral point of the power transformer, and the displacement voltage of the neutral point reflects to the open triangular winding of the voltage transformer, so that the open triangular winding of the voltage transformer appears overvoltage, the voltage transformer and a fuse thereof are possibly burnt, relay protection misoperation is possibly caused, and even relay protection equipment and electric quantity measuring equipment are damaged.
The ferromagnetic resonance of the voltage transformer is caused by a plurality of reasons, and the phenomenon of huge inrush current and iron core magnetic saturation is generated in a certain phase or two-phase winding of the transformer due to the sudden switching-on of a no-load bus or a power transmission line including the transformer; secondly, due to lightning stroke or other reasons, instantaneous single-phase arc grounding occurs in the circuit, so that other two phases are instantly raised to a line voltage, and a fault phase is instantly restored to a phase voltage after the grounding disappears, so that the transient exciting current is sharply increased and the magnetic saturation of an iron core is caused; third, the core may also be magnetically saturated due to an overvoltage momentarily transferred from another winding.
Generally, the ferromagnetic resonance of fractional harmonics and high frequencies is easy to distinguish, and can be distinguished only by calculating the harmonic quantity in voltage and adopting excessive criteria; however, the power frequency ferroresonance is complex, and when the power frequency ferroresonance is performed, the three-phase winding voltage of the voltage transformer still mainly uses the power frequency, and as a result, the three-phase fundamental voltage and the fundamental zero-sequence voltage may have various numerical relationships, such as one-phase slight saturation, two-phase slight saturation, one-phase serious saturation and two-phase serious saturation. The current method for judging the power frequency ferromagnetic resonance of the voltage transformer mainly adopts the following comprehensive criteria:
(1) if the three-phase fundamental voltage is increased, judging that power frequency ferroresonance occurs;
(2) if the voltage of one phase of fundamental wave is reduced and the voltages of the other two phases of fundamental waves are increased and exceed the line voltage, determining that power frequency ferromagnetic resonance occurs;
(3) if the voltage of one phase of fundamental wave is increased but not equal to 1.5 times of the rated voltage, and is in phase with the zero sequence voltage of the fundamental wave, and the voltages of the other two phases of fundamental waves are reduced and equal, the power frequency ferromagnetic resonance is judged to occur;
(4) if one phase voltage is reduced but not equal to 0 and is opposite to the zero sequence voltage of the fundamental wave, and the other two phases of fundamental wave voltages are increased and equal, the power frequency ferromagnetic resonance is judged to occur.
The method has the advantages of covering the situations of various power frequency ferromagnetic resonances, and has the defects of more classifications, complex criterion and need of judging the phase relation (in-phase or reverse phase) between a certain phase fundamental wave voltage and fundamental wave zero sequence voltage. In addition, in fact, when the voltage transformer has power frequency ferroresonance, the three-phase winding of the voltage transformer has unsaturation, saturation and saturation degree which cannot be completely the same, so that there are some conditions that the above method does not consider, for example, because the saturation degree of the two-phase winding is not consistent, there is a case that: one phase fundamental wave voltage is low, the other phase fundamental wave voltage is high and does not exceed the line voltage, and the third phase fundamental wave voltage is high and exceeds the line voltage. Therefore, the conventional method has disadvantages.
Disclosure of Invention
The purpose of the invention is: the method for judging the power frequency ferromagnetic resonance of the voltage transformer is provided, so that the judgment accuracy is further improved, and the defects of the conventional method are overcome.
The technical scheme adopted by the invention is to adopt the following steps to judge:
step one, sampling three-phase voltage of a voltage transformer;
step two, calculating three-phase fundamental voltage
And fundamental zero sequence voltage
Step three, if the fundamental wave zero sequence voltage effective value U0Greater than set fixed value threshold U0,setEntering the step four; otherwise, returning to the first step;
step four, judging whether the fault is a ground fault, if so, returning to the step one; otherwise, sending out power frequency ferroresonance alarm after short time delay.
Further, the fixed threshold U in step three0,setThe value range of (1) is 5% -30% of the rated phase voltage effective value, and the default value is 10% of the rated phase voltage effective value.
Further, the method for judging the ground fault in the fourth step adopts the following steps:
(1) the average value of the phase potential was calculated according to the following formula:
(2) the voltage radius 1 and the voltage radius 2 are calculated according to the following formulas:
(3) whether the following inequality is true is judged:
in the formulae 1, 2 and 3,
is a three-phase fundamental voltage; the symbol | | | is a modulus representing the calculated phasor; eavgIs the average value of the phase potentials; k1The coefficient is a value, the value range is 0.400-0.475, and the default value is 0.450; k2The coefficient is another coefficient, the value range is 0.525-0.600, and the default value is 0.550; er1Is the desired voltage radius 1; er2Is the desired voltage radius 2;
is a three-phase fundamental voltage
The subscript ph indicates phase, ph A, B or C indicates phase a, phase B or phase C, respectively;
is the fundamental zero sequence voltage;
if equation 3 holds true for any of the phases a, B, and C, it is considered that a ground fault has occurred.
Further, the short delay time range in the fourth step is 10ms to 10s, and the default time is 20 ms.
The method is simultaneously suitable for ungrounded systems and arc suppression coil grounded systems.
The invention has the beneficial effects that:
(1) compared with the prior art, the method reduces the complexity of classification judgment, does not need to judge the phase relation between certain phase fundamental wave voltage and fundamental wave zero sequence voltage, and is simpler;
(2) power frequency ferroresonance presents a situation: one phase fundamental wave voltage is low, the other phase fundamental wave voltage is high and does not exceed the line voltage, and the third phase fundamental wave voltage is high and exceeds the line voltage; compared with the prior art, the method can also cover the situation, and the method is more perfect;
(3) compared with the prior art, the method is simultaneously suitable for the mode that the neutral point of the generator is not grounded and is grounded through the arc suppression coil, and the application range is wider.
Drawings
FIG. 1 is a schematic flow diagram of the process of the present invention.
FIG. 2 is a three-phase fundamental voltage
And fundamental zero sequence voltage
Phasor diagram of (a).
Detailed Description
For the purpose of illustrating the method of the present invention, there is shown in the drawings and will herein be described by way of example specific embodiments of the invention. First, a 10kV power distribution network system is taken as an example to illustrate the basic principle of the invention.
A 10kV power distribution network system, electricityThe voltage transformation ratio of the voltage transformer is 10kV/100V, and the system is not grounded; A. b, C the three-phase power supply potential only considers the power frequency fundamental wave, respectively
(secondary values after transformation by the voltage transformer are the same below), A, B, C the three-phase power supply potential is symmetrical, namely the effective values are equal and the phase angles are different by 120 deg; let A, B, C three phases have the same capacitance to ground, all of which are C0. The fundamental voltage of A, B, C three phases relative to the ground measured by the voltage transformer is normal without fault when the line impedance is ignored
(secondary value after transformation by voltage transformer, the same applies below), fundamental wave zero sequence voltage at this time
(the secondary value after the transformation by the voltage transformer, the same applies below) is 0V. When a single-phase grounding fault occurs to a certain phase of the power grid, if the resistance value of the grounding transition resistor is R, A, B, C fundamental wave voltages of three phases to the ground are changed, and at the moment, the fundamental wave zero sequence voltage is as follows:
wherein,
is the supply potential of the faulted phase, the subscript ph denotes the faulted phase, ph ═ a, B or C; c0Is the capacitance per phase; omega is the angular frequency corresponding to the power frequency; and R is the resistance value of the grounding transition resistor.
Similarly, in the 10kV power distribution network system, the voltage transformation ratio of the voltage transformer is 10kV/100V, if the neutral point of the system is grounded through the arc suppression coil, and the equivalent inductance value of the arc suppression coil is L, when a single-phase ground fault occurs in a certain phase of the power grid, and the resistance value of the grounding transition resistor is R, the fundamental voltages of A, B, C three phases to the ground are all changed, and at this time, the fundamental zero-sequence voltage is:
wherein,
is the supply potential of the faulted phase, the subscript ph denotes the faulted phase, ph ═ a, B or C; c0Is the capacitance per phase; omega is the angular frequency corresponding to the power frequency; l is equivalent inductance of the arc suppression coil; and R is the resistance value of the grounding transition resistor.
When power frequency ferroresonance appears in voltage transformer, some unsaturations of voltage transformer's three-phase winding, some saturation, often saturation degree is not the same completely moreover, consequently can lead to A, B, C three fundamental voltage to ground all to change, and lead to appearing fundamental zero sequence voltage. In most cases, the fundamental wave zero sequence voltage during power frequency ferroresonance and the fundamental wave zero sequence voltage during single-phase earth fault have obvious difference in amplitude and phase, so the basic idea of the method of the invention is as follows: if the characteristics (amplitude and phase) of the fundamental wave zero-sequence voltage are not the characteristics of the fundamental wave zero-sequence voltage in the single-phase earth fault, the power frequency ferroresonance is judged to occur.
The invention provides a method for judging power frequency ferromagnetic resonance of a voltage transformer, which adopts the technical scheme that the method adopts the following steps:
step one, sampling three-phase voltage of a voltage transformer;
step two, calculating three-phase fundamental voltage
And fundamental zero sequence voltage
Step three, if the fundamental wave zero sequence voltage effective value U0Greater than set fixed value threshold U0,setEntering the step four; otherwise, returning to the first step;
step four, judging whether the fault is a ground fault, if so, returning to the step one; otherwise, sending out power frequency ferroresonance alarm after short time delay.
Further, in step IIIThe fixed value threshold U0,setThe value range of (1) is 5% -30% of the rated phase voltage effective value, and the default value is 10% of the rated phase voltage effective value. For example, for the 10kV distribution network system, if the rated phase voltage effective value (secondary value after transformation by the voltage transformer, the same applies hereinafter) is 57.74V, U may be set0,set＝10％×57.74V＝5.774V。
Further, the method for judging the ground fault in the fourth step adopts the following steps:
(1) the average value of the phase potential was calculated according to the following formula:
wherein,
is the three-phase fundamental voltage calculated in the second step; the symbol | | | is a modulus representing the calculated phasor; eavgIs the average value of the phase potentials; for the 10kV power distribution network system, the line voltage is unchanged when single-phase earth fault occurs or the power frequency ferromagnetic resonance of the voltage transformer occurs, and E can be obtained through the formula 1 under the condition of rated voltageavg＝57.74V。
(2) The voltage radius 1 and the voltage radius 2 are calculated according to the following formulas:
wherein E isavgIs the average value of the phase potential calculated in the step (1); k1The coefficient is a value, the value range is 0.400-0.475, and the default value is 0.450; k2The coefficient is another coefficient, the value range is 0.525-0.600, and the default value is 0.550; er1Is the desired voltage radius 1; er2Is the desired voltage radius 2; for the 10kV power distribution network system, K is taken1＝0.450、K2When the rated voltage is 0.550, E can be obtained by the equation 2r1＝0.450×57.74V＝25.983V、Er1＝0.550×57.74V＝31.757V。
(3) Whether the following inequality is true is judged:
wherein,
is the three-phase fundamental voltage calculated in the second step
The subscript ph indicates phase, ph A, B or C indicates phase a, phase B or phase C, respectively;
the fundamental wave zero sequence voltage calculated in the second step; the symbol | | | is a modulus representing the calculated phasor; er1Is the voltage radius 1 calculated in the step (2); er2Is the voltage radius 2 calculated in the step (2).
If equation 3 holds true for any of the phases a, B, and C, it is considered that a ground fault has occurred.
Still taking the 10kV power distribution network system as an example, a single-phase earth fault example of a certain time is three-phase fundamental voltage
Respectively as follows:
the fundamental wave zero sequence voltage is:
calculated from equation 1: eavg60.6V. Get K1＝0.450、K20.550, calculated from equation 2: er1＝27.27V、Er233.33V. Further calculation shows that:
since the phase a satisfies the requirement, it can be determined as a ground fault by the equation 3.
Still taking the 10kV power distribution network system as an example, a power frequency ferromagnetic resonance example appears in a certain voltage transformer, and three-phase fundamental voltage
Respectively as follows:
the fundamental wave zero sequence voltage is:
calculated from equation 1: eavg59.94V. Get K1＝0.450、K20.550, calculated from equation 2: er1＝26.973V、Er232.967V. Further calculation shows that:
according to the formula 3, the A phase, the B phase and the C phase can not meet the requirements, so that the condition that the phase is not a ground fault can be judged, and a power frequency ferromagnetic resonance alarm is sent out after short time delay.
The short delay time range in the fourth step is 10 ms-10 s, and the default is 20 ms.
The method is simultaneously suitable for ungrounded systems and arc suppression coil grounded systems.
The above embodiments are merely illustrative of the technical ideas of the present invention, and the technical ideas of the present invention can not be used to limit the scope of the present invention.
Claims (3)
1. The method for judging the power frequency ferromagnetic resonance of the voltage transformer is characterized by comprising the following steps of:
step one, sampling three-phase voltage of a voltage transformer;
step two, calculating three-phase fundamental voltage
And fundamental zero sequence voltage
Step three, if the fundamental wave zero sequence voltage effective value U0Greater than set fixed value threshold U0,setEntering the step four; otherwise, returning to the first step;
step four, judging whether the fault is a ground fault, if so, returning to the step one; otherwise, sending power frequency ferromagnetic resonance alarm by short time delay; the specific method for judging the ground fault comprises the following steps: respectively calculating the modulus of the phasor sum of the fundamental wave zero sequence voltage and each phase of fundamental wave voltage, calculating the average value of three-phase potential, and judging as the ground fault when the modulus value of the phasor sum of any phase is in a set range;
the method for judging the ground fault comprises the following steps:
(1) the average value of the phase potential was calculated according to the following formula:
(2) the voltage radius 1 and the voltage radius 2 are calculated according to the following formulas:
(3) whether the following inequality is true is judged:
in the formulae 1, 2 and 3,
is a three-phase fundamental voltage; the symbol | | | is a modulus representing the calculated phasor; eavgIs the average value of the phase potentials; k1Is a coefficient, and the value range is 0.400-0.475; k2The other coefficient is a value range of 0.525-0.600; er1Is the desired voltage radius 1; er2Is the desired voltage radius 2;
is a three-phase fundamental voltage
The subscript ph indicates phase, ph A, B or C indicates phase a, phase B or phase C, respectively;
is the fundamental zero sequence voltage;
if equation 3 holds true for any of the phases a, B, and C, it is considered that a ground fault has occurred.
2. The method for determining power frequency ferroresonance of a voltage transformer as claimed in claim 1, wherein said constant threshold U in step three0,setThe value range of the phase-change voltage is 5 to 30 percent of the effective value of the rated phase voltage.
3. The method for determining power frequency ferroresonance of a voltage transformer as claimed in claim 1, wherein the short delay time range in the fourth step is 10ms to 10 s.
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